Concentration of gold during the uncovering of titanium-bearing placers of Sikhote-Alin with a mixture of hydrodifluoride and ammonium sulfate

The possibility of concentrating gold during the processing of titaniumbearing placers of ultrabasites of the Sikhote-Alin orogenic belt has been studied. It has been shown that during electromagnetic separation, gold is collected in the non-magnetic fraction. To uncover mineral raw materials, it is proposed to use a mixture of hydrodifluoride and ammonium sulfate. It has been established that when heated with the indicated reagents in the temperature range up to 200 оC, fluoridation of the components included in the mineral raw materials occurs with formation of simple and complex fluorides. A further increase in temperature (in the range of 300–360 оC) as a result of interaction with the resulting ammonium bisulfate leads to their conversion into soluble double sulfates. It has been determined that ammonium hexafluorosilicate (NH4)2SiF6 does not react with NH4HSO4, but passes into the gas phase and is collected in a sublimator together with ammonium hydrodifluoride. Thermogravimetric study of sublimation showed that these two products can be separated by selective distillation of NH4НF2 from this mixture, which facilitates the regeneration of this reagent and its further use in circulation. It has been shown that aqueous leaching of the reaction product allows almost all titanium and the bulk of iron to be transferred into solution in the form of double salts that are highly soluble in water, while gold can be concentrated in an insoluble sediment more than 400 times along with NH4НF2-resistant minerals. The influence of the NH4НF2 content in the reaction mixture on the degree of gold concentration is shown. Ways have been proposed for isolating the components of mineral raw materials in the form of commercial products and regenerating the fluorinating reagent.The authors express their gratitude to D. G. Epov (Candidate of Chemical Sciences) for his assistance in conducting experimental studies.The work was carried out with financial support from the Russian Science Foundation (RSF grant No. 23-17-00093) and within the framework of the state assignment for the Institute of Chemistry, Far Eastern Branch of the Russian Academy of Sciences, topic FWFN-2022-0001.